Hämähäkkikalvo

Hämähäkkikalvo refers to the arachnoid mater, one of the three meninges that surround the brain and spinal cord. The meninges are protective membranes that lie between the skull and the brain, and between the vertebrae and the spinal cord. The three layers of meninges are, from outermost to innermost: the dura mater, the arachnoid mater, and the pia mater.

The arachnoid mater is a thin, wispy, and transparent membrane located just beneath the dura mater. It derives its name from its spiderweb-like appearance, due to the delicate, thread-like trabeculae that extend from its inner surface to connect with the pia mater. These trabeculae are composed of connective tissue. The space between the arachnoid mater and the pia mater is called the subarachnoid space. This space is filled with cerebrospinal fluid (CSF), which acts as a cushion for the central nervous system and plays a role in nutrient transport and waste removal.

Blood vessels that supply the brain and spinal cord also travel through the subarachnoid space. The arachnoid mater, along with the pia mater, forms the leptomeninges. Protrusions of the arachnoid mater through the dura mater into the venous sinuses are known as arachnoid granulations or villi, which are crucial for the reabsorption of cerebrospinal fluid back into the bloodstream.